When they accidentally fell asleep together on the Argo 2. It was a sweet moment for both of them and then it was turned into something funny for everyone else. Also, when Percy refused to let her go and fell into Tartarus for her.
Also @Aiden-1089 yes that was a pretty good moment too
I voted for the first one. Though, I think the dance in TTC was also kinda cute, when both of them were young and innocent. The art for that is amazing, actually.
Oh and I just saw something on Pinterest that reminded me - Annabeth had like emailed him a picture of her on vacation and he printed it out and taped it into his notebook then carried it around with him everywhere in school